The co-orbital asteroids of Jupiter, also known as the 'Trojan asteroids'. The prograde asteroids are shown in white, and 2015 BZ509 (with a trail, shown in green) appears later. The planets and asteroids have been enlarged for visibility. Click above for a larger version.

In the March 30 2017 issue of the
journal Nature, astronomers
reveal that an as-yet-unnamed
rare asteroid
with the provisional designation 2015 BZ509 (nicknamed 'BZ') travelling in the
opposite direction to all the planets and 99.99% of the other
asteroids in our Solar System — a state referred to
as retrograde
motion— is also safely sharing the orbital space of the giant planet
Jupiter.

There are about 6000 other asteroids which share Jupiter's orbit
space with it. Called the 'Trojan
asteroids', they co-exist easily with the giant Jupiter because
they go around in the same direction, called prograde
motion. If the Solar System were a giant race track around the Sun,
the planets would be like monster trucks, and the asteroids like
ridiculously tiny clown cars. Though all this traffic is inherently
dangerous, collisions are usually avoided because the planets and
asteroids go around the track in the same direction.

What's strange about BZ is, first, that it goes around the
track in the opposite or 'retrograde' direction to most all the
others. But it is not the only asteroid going the opposite way. A few others do,
so this makes BZ unusual but not unique. The second and stranger thing is that
BZ is also playing a cosmic game of 'chicken' with the giant
planet Jupiter. The other retrograde asteroids tend to remain away
from the planets. This makes sense: if a clown car is going to
survive going the wrong way around the track, best to stay away from
the big trucks. But BeeZed actually shares the same lane as
Jupiter, and it does so while going around the track in the
opposite direction. This is not what one would expect to be a
very long-lived situation, but this study shows that BZ has
done so safely for at least tens of thousands of 'laps', avoiding
the monster truck by weaving in and out of the planet's path every
time they pass. Jupiter's gravity actually helps it maintain this
state by nudging it a little every lap to help keep
BZ synchronized. 2015 BZ509 is the first asteroid known to have
this relationship with any of the planets. Calculations show it will
continue to safely navigate its unusual path for the next million
years at least.

2015 BZ509 maintains a stable relationship with respect to Jupiter not
simply by luck but —perhaps surprisingly— through the
effect of Jupiter's gravity. BZ passes once inside and once outside
Jupiter each time they orbit the Sun, and the two gravitational tugs
that Jupiter gives the asteroid cancel out, giving BZ opposing
'nudges' that keep it on track. The link between the asteroid and
Jupiter is a distinct but gentle one: BZ nevers gets closer to Jupiter
than the Earth does to the Sun: the closest approach between the two
is 176 million kilometers (109 million miles)
centre-to-centre. Ironically, BZ would be more likely to crash into
Jupiter if that planet had no gravity at all, because without the
gravitational nudges, it would gradually drift out of sync with that
planet.

That asteroids could theoretically be in such a unusual state, called a
retrograde co-orbital resonance was first noted by Dobrovolskis (2012), and an excellent theoretical understanding of such behaviour
was soon worked out by Helena Morais and Fathi Namouni in a series
of research papers in
2013a,
2013b,
2015, and
2016.
But 2015BZ509 is the first real physical object
in this unusual state. How it got there remains a mystery.

Images of 2015 BZ509
obtained at the Large Binocular Telescope Observatory (LBTO)
that established its retrograde co-orbital nature. The LBTO has
two 8.4 meter-wide main mirrors side-by-side, hence the two
images. The bright stars and the asteroid (circled in yellow)
appear black and the sky white in this negative image. What are
those weird white dots, spots and stripes? They are imaging
artifacts in these raw images. Click for a larger view.

This asteroid may in fact be an inactive
icy comet nucleus,
rather than a
rocky asteroid. Comets
are much more often on retrograde orbits to begin
with: Halley's
Comet is just one example of a comet on a retrograde orbit.
However, observations of BZ have so far failed to show any
indication of comet-like behaviour, such as a tail. Complicating the
picture is that comets only develop tails when the icy comet nucleus
gets close enough to the Sun for its ice to start to
vaporize. Though BZ has been observed when it was at its closest to
the Sun, it does not get particularly close to our star even at its
closest, so it may never receive enough sunlight to form a tail even
if it had any icy composition. The answer to the origin and nature
of this object will have to await further study.

Asteroid 2015 BZ509 always remains in the vicinity of Jupiter's
orbit, and does not approach the Earth. The asteroid is not
considered hazardous to our planet.

More on Co-orbital Asteroids

"Co-orbital asteroids" are so-named because they can share a planet's
orbit with it, and they come in a wide variety of strange
relationships with their companion planet. For more information on
co-orbital asteroids, see the links at the bottom of this page.

How was the nature of 2015 BZ509 discovered?

Asteroid 2015 BZ509 was discovered by the Panoramic Survey Telescope
And Rapid Response System (Pan-STARRS) in 2015. At the time, its
orbit was not sufficiently well-known to determine its behaviour. But
it appeared to be suspiciously close to Jupiter's co-orbital zone, so the
authors of this paper tracked it down and obtained further
observational data using the Large
Binocular Telescope Observatory in Arizona. From the analysis of
these measurements, the retrograde co-orbital nature of this strange
asteroid was determined.

The green ellipse is the orbit of 2015 BZ509 around the Sun, the
blue orbit is Jupiter's, and the other planets are shown in
grey. The green dots represent the path of the asteroid as seen
from the reference frame of Jupiter. The Sun is at the center of
the frame. Jupiter and the asteroid are labelled. Click on the image to the left to see the video.

The path of asteroid 2015 BZ509 relative to Jupiter is shown in green
for several orbits. The Sun is the yellow sphere, Jupiter is the large
planet in the foreground centre, and the light blue circle traces out
Jupiter's orbit around the Sun. Jupiter is moving counter-clockwise
(to the right in this view). Note however, that the camera moves along
with it to track its motion so while it is moving around the Sun, it
appears stationary with respect to the camera. Click on the image to
the left to see the video.

A view from across Jupiter's orbit, looking toward that giant
planet. The prograde Trojan asteroids (white) of Jupiter along with
asteroid 2015 BZ509 (green). The Trojans asteroids are white spheres
which mill about ahead of and behind Jupiter along its orbit. The
light blue circle traces out Jupiter's orbit around the Sun. The other
planets and their orbits are shown in white. Jupiter is moving
counterclockwise (to the left in this view), but once again the camera
moves along with it to track its motion so it remains in the centre of
the field of view. A few stray non-Trojan asteroids are also
included. Click on the image to the left to see the video.

The Nature journal article
can be accessed at
this link.
Springer, the publisher of Nature, also kindly provides a SharedIt
link: if you don't have access to a subscription to Nature, you can
still view the
article here. A commentary
by Morais and Namouni is also available.

Interactive 3D visualizations of the orbit. These files are Wolfram
CDF (Computable Document Format) objects generated with Mathematica 10
on a Raspberry Pi 3 computer. Ephemeris computations for them were
done with the JPL Horizons system accessed through telnet
ssd.jpl.nasa.gov 6775 . Wolfram CDF player (or
equivalent) is needed to view them. The authors of these files take no
responsibility for their playability, nor for the results of any
interaction of them or of the player with user’s computers. When first
displayed, some details may be missing. Click on the image to make
these clear and to change the point of view by dragging. Magnification
may help in seeing details.

XYZorbit.cdf The orbits of Jupiter and 2015 BZ 509 in the current epoch. The Sun is at the center. One orbit of each is shown: they have roughly the same period of 11.86 years.

XYZlibration.cdf The orbits of Jupiter and 2015 BZ 509 are presented over one half libration cycle of the asteroid (2017-2342), during which the orbit of Jupiter does not change significantly.

Relative orbits: the orbit is shown in the frame that rotates with Jupiter. In this frame the asteroid
describes a complex three-dimensional pattern but the view is scaled so that Jupiter is fixed at position
(1,0,0). The Sun is at the center. The asteroid’s relative orbit moves through time due to libration. The
basketweave pattern arises from the changes taking place mostly on the close passes to Jupiter.

relOrbit.cdf The scaled orbit of 2015 BZ 509 relative to Jupiter is shown, similar to that in Figure 1 in the paper.

relLibration.cdf The scaled orbit of 2015 BZ 509 relative to Jupiter is shown over one half libration cycle. The first orbit starts in 2017 and is shown in orange; the last ends in 2342 and is in green.
Intervening orbits are shown in blue.

relLibrationColor.cdf The scaled orbit of 2015 BZ 509 relative to Jupiter is shown, over one half libration cycle. Z R values are color coded from the most negative value possible in blue, to the highest value possible in red. Z R is the perpendicular distance to 2015 BZ 509 from Jupiter’s orbital plane.